Theory of commensurate-incommensurate transitions and melting in freon (CF4) adsorbed on graphite

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Abstract
The transitions between fluid, incommensurate, striped, and commensurate (2×2) phases in freon adsorbed on graphite are analyzed in terms of two types of topological defects: walls and dislocations. We find that a commensurate (2×1) phase separates the striped phase from the fluid phase and the (2×2) phase. The resulting phase diagram includes melting transitions of the Kosterlitz-Thouless type as well as the Nelson-Halperin type and a Heisenberg transition with "corner-type" anisotropy. Specific experiments are suggested to test several recent theories on two-dimensional systems.